As a system for making a diagnosis on an inner portion of a body cavity of a patient, an electronic endoscope system is generally known, and has been put into practical use. An electronic endoscope having a function of illuminating a subject with light via a narrow band filter which allows light having a wavelength band, within which a particular organism exhibits a high absorption property, to pass therethrough, and generating a spectral image highlighting the particular organism by receiving a scattered light component from the subject is known. However, in the electronic endoscope system of this type, the observable wavelength band is fixed. Therefore, there is a problem that a normal color image cannot be obtained, and it is impossible to conduct a comparative diagnosis between a normal color image and a spectral image.
For this reason, for example, Japanese Patent Provisional Publication No HEI 1-297042A (hereafter, referred to as patent document 1) proposes an electronic endoscope system which makes it possible to conduct a comparative diagnosis. Specifically, the electronic endoscope system described in patent document 1 is configured to control the wavelength band of illumination light by switching the bandpass filter turret arranged on an illumination light path and to selectively generate a spectral image and a normal color image. By shooting both of these images and then making a comparison between these images, it becomes easier to understand the relationship between a particular organism and another organism, and therefore advantages that the accuracy of a diagnosis is enhanced can be expected.